Using a level-oriented model for verification of asynchronous circuits helps users to easily construct formal models with high readability or to naturally model datapath circuits. On the other hand, in order to use such a model on large circuits, techniques to avoid the state explosion problem must be developed. This paper first introduces a level-oriented formal model based on time Petri nets, and then proposes its partial order reduction algorithm that prunes unnecessary state generation while guaranteeing the correctness of the verification.